The present invention relates to a membrane and a method of preparing the membrane, the membrane being particularly, but not exclusively, useful in producing synthetic gas for use in Fischer-Tropsch gas-to-liquids production in the oil and gas exploration industry or for producing hydrogen for use as a fuel.
While offshore oil production has risen slightly in recent years, natural gas (which mainly consists of methane) production has seen a marked increase. Natural gas is often extracted during the extraction of liquid hydrocarbons, such as oil, from the ground and is often undesirable due to the lack of infrastructure to transport the natural gas to an onshore location. The lack of infrastructure can be explained by the physical nature of natural gas which makes it difficult to transport safely and/or efficiently in its basic gaseous state. As a result the natural gas is often flared (ignited) causing economic waste and environmental concern. It would therefore be desirable to either convert the natural gas into some other substance which can be transported easily, or transport the natural gas in a liquid state. In this way, new field development will be more financially viable through the use of the extensive infrastructure and technology already in place in the offshore industry for transporting liquid hydrocarbons.
It is known to transport natural gas as a Liquid Natural Gas (LNG) in specifically constructed containers onboard vessels which have been adapted for such purposes. However, this has many disadvantages including; the need for expensive pressurising equipment which is difficult to scale down to suit smaller production fields, loss of gas during transportation (“boil-off”), danger posed in transit to vessel and crew by high pressure, highly flammable gases and the requirement to depressurise the LNG into a usable gaseous state at the customer end.
It is considered that a better way of utilising offshore produced natural gas (CH4) is to convert it, on or in close proximity to the offshore production platform, into synthetic gas (syngas) which can in turn be used to produce gases, fluids and chemicals such as methanol, ammonia and importantly, crude oil that can be readily pumped through the same pipelines as the produced oil. Syngas comprises a mixture of carbon monoxide (CO) and hydrogen (H2).
By way of background information to the reader, conversion of syngas to liquid hydrocarbon is a chain growth reaction between carbon monoxide and hydrogen on the surface of a heterogeneous catalyst. The catalyst is either iron or cobalt based and the reaction is highly exothermic. The temperature, pressure, and catalyst determine whether a light or heavy syncrude is produced. For example at 330° C. mostly gasoline and olefins are produced whereas at 180° C. to 250° C. mostly diesel and waxes are produced. There are two main types of Fischer-Tropsch reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurised boiling water. In large plants, several reactors arranged in parallel may be used, presenting energy savings. Another process uses a slurry reactor in which pre-heated syngas is fed into the bottom of the reactor and distributed into the slurry which consists of liquid wax and catalyst particles. As the syngas bubbles upwards through the slurry, it is diffused and converted into more wax by the Fischer-Tropsch reaction. The heat generated is removed through the reactors cooling coils where steam is generated for use in the process. Again by way of background information to the reader, this is shown in FIG. 7.
Thus if methane (or other gaseous hydrocarbons) could be converted to syngas and thereafter to liquid hydrocarbons, the transportation costs and difficulties outlined above would be mitigated.
Synthesis gas can be made by partial oxidation of methane (although it is more usually made by the reaction of methane with steam under pressure.)
A major safety problem with the partial oxidation of methane arises because methane and air (or oxygen) should be fed into the reactor at the same time and therefore there is the danger of an explosion.
It is known in the art that a reactor with relatively dense ceramic membranes that conduct oxygen can be used for syngas production (e.g. WO 98/48921 and WO 01/93987). These membranes generate syngas by avoiding direct contact between the oxygen and hydrocarbon feed, but this necessitates the use of very high temperatures in order to achieve the necessary oxygen flux. Moreover, being dense means that the membrane has to be as thin as possible, resulting in brittleness and crack formation, loss of efficiency and reduced operating service life. In some cases the membrane would need to be so thin that it would be unable to support its own weight and therefore impossible to use in practice.
Cost effective natural gas (methane) conversion to syngas for gas-to-liquids production would therefore be an important commercial development.
Hydrogen can be used as a clean fuel. However, the amount of hydrogen that can be produced by using renewable natural energy sources such as solar, wind, and hydro-power is currently not sufficient to satisfy demand. The utilisation of natural gas and/or the production of hydrogen from natural gas seen to be a viable alternative and the most realistic solution at least in the first half of this century [1, 2].
An example of progress in the widespread utilisation of natural gas involves the development of small co-generation system using the micro-gas turbine. In addition, fuel cells are expected to be a highly-efficient power generating system. The fuel cells are anticipated to be deployed in residences in addition to the installation in electrical vehicles. Home-use of fuel cells can provide hot-water and electricity, simultaneously. To commercialise the stationary fuel cells, it is necessary to establish alternative hydrogen generation technology.